1. Field of the Invention
The present invention relates to a vehicle suspension system for a steerable wheel. More particularly, it relates to a vehicle suspension system for a steerable wheel which makes it easy to provide an imaginary kingpin axis, improves handling safety by setting up imaginary links and minimizes the variation ratio of the height of a roll center to enhance a free layout degree with respect to the change in camber and tread, and especially ensures maximum effective volume in an engine room by minimizing the space occupied by a suspension system.
2. Description of Related Art
In general, a vehicle suspension system for a steerable wheel connects an axle and a vehicle body to each other, and absorbs vibrations and impacts from a road surface during the vehicle's running, such that safety of the vehicle can be obtained, and ride comfort is improved.
To absorb impacts, a vehicle suspension system should be connected flexibly in a vertical direction, and to endure driving force, braking force and centrifugal force generated at a wheel during the vehicle's turning, a vehicle suspension should be connected firmly in a horizontal direction.
The vehicle suspension system is classified structurally as a suspension system integrated with an axle and an independent suspension system. While a suspension system integrated with an axle is applied to a large vehicle like buses, trucks, and rear axles of passenger cars, an independent suspension system is applied mainly to an axle of a passenger car. To improve ride comfort and running safety, an independent suspension is bisected to act independently.
Among these suspension systems, the present invention relates to an independent suspension system.
There are many kinds of suspension systems in independent suspension systems. As a suspension system applied to a steerable wheel, Macpherson type and Wishbone type are widely applied.
A vehicle suspension system for a steerable wheel is designed to control a position of a wheel with respect to the vehicle body to obtain a better positioning in a vehicle operation as well as absorb vibrations or impacts. The axle is thereby prevented from directly transmitting impact or vibration from a road surface to the vehicle body, so that optimal handling safety and running safety can be obtained.
The front wheel is mounted to be changeable in its direction from right to left or from left to right, centering the kingpin, and is also mounted with a geometrical angle to meet the requisite for the front suspension.
Mounting the front wheel with the geometry angle is called wheel alignment. The wheel alignment is determined by several elements, but the optimal operation of the front wheel is, however, achieved by the supplemental operation of the elements with respect to each other.
A kingpin inclination among the elements reduces handling force of a steering wheel with a camber, absorbs any impact generated from the vehicle motion and then increases restitution force of the steering wheel. The turning safety of the straight ahead position of the vehicle and the handling safety in the vehicle's turning position are thereby obtained.
The kingpin inclination is such that the kingpin is mounted with its central line lying at a predetermined angle with respect to the vertical line when the vehicle is viewed from the front.
An interval between the central line of the kingpin and a central line of the wheel is called offset. The offset is generally characterized as an offset at the wheel center and an offset at the ground.
The offset at the wheel center has an effect on the straight ahead characteristics of the vehicle when driving force and an engine brake are applied to the wheel.
The offset at the ground has an effect on the handling safety when the vehicle brakes and turns. If the offset at the ground is decreased, the sensitivity with respect to the steering is decreased, in the same manner as the handling safety is increased when the vehicle brakes and turns.
Vibrations have something to do with the vehicle safety, since rolling, pitching, yawing as well as bounce occur during the vehicle's driving. Only if these vibrations are absorbed softly, will ride comfort and safety be improved.
As the vibration of a vehicle is concerned, vibration generated when a vehicle is turning is defined as rolling, which means that a vehicle vibrates in the left-and-right directions, and rolling has an important effect upon turning safety.
The certain basic point where rolling occurs is called a roll center (RC). Rolling occurs because the centroid of a vehicle is higher than a roll center. Accordingly, the greater the change in a height of a roll center, the greater the center of gravity of a vehicle moves. Therefore, this phenomenon results in a greater slip angle, so the running safety and the handling safety get worse.
To ensure running safety, the variation ratio of the height of RC is desired to be maintained.
Considering the above, the prior suspension system is described in detail hereinafter.
FIG. 5A illustrates a general Macpherson type suspension system, which comprises a strut arm 113 formed integrally to a steering knuckle 111, provided elastically with a spring 114 at the upper end and a shock absorber 112, and a lower arm 115 attached to the steering knuckle 111 by a ball joint 116 and to the vehicle body by a hinge at the other end of the lower arm.
This suspension system has some advantages of simple structure including a small spring, light mass, and a small effective volume of an engine room. But in order to make the kingpin offset (a) little or minus, the upper supporting point 117 of the strut arm 113 is to be moved toward the engine room, or the lower end of the strut arm 113 connected to the ball joint 116 is displaced outside.
However, if the upper supporting point 117 of the strut arm 113 is displaced toward the engine room, the effective volume of an engine room is reduced and the kingpin angle (.alpha.) becomes excessively large only to have a bad effect on cornering ability of a vehicle.
If the lower end of the strut arm 113 in alignment with the ball joint 116 is displaced outside, it is actually impossible to reduce the kingpin offset (.alpha.) because it will interfere with a brake disk attached to a wheel.
FIG. 5B illustrates a general Wishbone type suspension system including upper and lower control arms 121, 122, a steering knuckle 123, a spring assembly 125 including a shock absorber 124 and ball joints 126, 127 connecting the upper and lower control arms 111, 122 to the steering knuckle 123.
In order to adjust the kingpin offset (.alpha.), the vehicle body side connection portion 128 of the upper control arm 121 should be displaced toward the engine room, or the ball joint 127 of the lower control arm 122 should be displaced outside.
But, in a Wishbone type suspension system, as well as Macpherson type suspension system, if the vehicle body side connecting portion 128 of the upper control arm 121 is displaced toward the engine room, the effective volume of an engine room is reduced and the kingpin angle (.alpha.) becomes excessively large to adversely affect turning ability of a vehicle, and if the ball joint 127 of the lower control arm 122 is displaced outside, it is actually impossible to reduce the kingpin offset because it will interfere with a brake disk attached to the wheel.